The present invention relates generally to filtration devices and more particularly to microfabricated filters constructed with permeable membranes. The present invention further relates to microfabricated shells constructed with such membranes for encapsulating microfabricated devices such as microelectromechanical structures (MEMS).
Filtration devices are extensively used in industrial applications, such as within the biomedical industry, for separating particles of specific sizes from a fluid. For these applications, required filtration device features typically include: relatively uniform pore sizes and distributions, pore sizes as small as the nanometer (nm) range, high throughput, and adequate mechanical strength.
Filter pore sizes in the nanometer range would allow biologically-important molecules to be mechanically separated on the basis of size. For instance, such pore sizes may be used to achieve the heretofore elusive goal of viral elimination from biological fluids.
Filters constructed with porous materials are known in the art. For instance, a porous polycrystalline silicon (polysilicon) plug for use as a filter is described by Anderson in "Formation, Properties, and Applications of Porous Silicon," Ph.D. Thesis, Dept. of Chemical Engineering, U.C. Berkeley, April 1991, and summarized in "Porous Polycrystalline Silicon: A New Material for MEMS," Journal of Microelectromechanical Systems," Vol. 3, No. 1, March 1994, pp. 10-18. The porous polysilicon plug is formed by depositing a layer of polysilicon on a substrate using low-pressure chemical vapor deposition (LPCVD) and then etching the polysilicon layer with an electrochemical anodization process to make it porous. The porous polysilicon provides pore features of about 0.3 micrometers (.mu.m) in width. The electrochemical etching process, however, requires an anodization apparatus, which is not typically used in standard microfabrication processes. Furthermore, the porous polysilicon plug is permeable only in a planar direction with respect to the substrate.
The permeability of thin layers (less than about 0.3 .mu.m thick) of polysilicon to hydrofluoric (HF) acid has been discussed by Judy et al. in "Polysilicon Hollow Beam Lateral Resonators," Proceedings of the IEEE Micro Electromechanical Systems Workshop, Fort Lauderdale, Fla., Feb. 1-10, 1993, pp. 265-71; by Monk et al. in "Stress-corrosion Cracking and Blistering of Thin Polysilicon Films in Hydrofluoric Acid," Materials Research Society Symposium Proceedings, Vol. 308, San Francisco, Calif., May 1993, pp. 641-6; and by Chonko et al. in "The Integrity of Very Thin Silicon Films Deposited on SiO.sub.s," The Physics and Chemistry of SiO.sub.S and the Si--SiO.sub.s Interface 2, edited by C. R. Helms, Plenum Press, New York, 1993, pp. 357-62. However, these references are not directed to the use of thin layers of polysilicon as filters.
Microfabricated shells are used to encapsulate microfabricated devices such as MEMS. MEMS include devices such as micro-resonators and inertial sensors. The shells provide a hermetic, low-pressure environment that is essential for achieving a high quality (Q) factor and low Brownian noise in the operation of MEMS.
Microfabricated shells may be fabricated by etching a sacrificial layer disposed beneath a frame layer, thus forming a cavity, as described by Lin in "Selective Encapsulation of MEMS: Micro Channels, Needles, Resonators and Electromechanical Filters," Ph.D. Thesis, ME Department, University of California, Berkeley, Berkeley, Calif., December 1993. In this technique, etch holes are formed through the frame layer to allow an etchant to pass into the shell and etch the sacrificial layer. The etch holes are subsequently closed to hermetically seal the shell by depositing a sealant over the frame layer.
The etch holes are placed around the perimeter of the frame layer to minimize the amount of sealant passing through the etch holes and depositing on the encapsulated microfabricated device. Deposition of sealing film on the microfabricated device is undesirable since it may alter the device's characteristics. However, this placement of the etch holes increases the time required to etch the sacrificial layer due to the increased distance the etch is required to travel to remove the sacrificial layer. Long etch times are undesirable since long-term exposure to hydrofluoric acid is damaging to polysilicon structures which may be present in the microfabricated device. As a result, the width of shells must be limited in order to keep the etch times reasonable.
The use of permeable polysilicon for fabricating microfabricated shells is mentioned by Judy in "Micromechanisms Using Sidewall Beams," Ph.D. Thesis, EECS Dept., U.C. Berkeley, May 1994 and by Lin in his 1993 Ph.D. Thesis mentioned above. However, neither reference discloses any details of a structure or fabrication process for incorporating permeable polysilicon in such shells.
Accordingly, it is an object of the present invention to provide filters having a pore width as small as the nanometer range, yet also having a pore length as small as the tenths of a micrometer range to maximize throughput.
An additional object of the present invention is to provide filters that have a high mechanical strength.
A further object of the present invention is to provide methods for the construction of such filters using standard microfabrication processes.
Another object of the present invention is to provide shells that minimize the damage incurred by the encapsulated microfabricated device during the fabrication of the shell without restricting the width of the shell.
Yet another object of the present invention is to provide methods for the construction of such a shell.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the claims.